LONDON—Mining company
Rio Tinto
RIO 2.16%
PLC has privately assured Guinea that its recently filed lawsuit against Brazilian rival
Vale SA
isn't a secret move to reclaim control of a valuable African iron-ore project, according to a letter seen by The Wall Street Journal.

Guinean government officials had feared that Rio Tinto's suit would discourage rival companies from taking part in any future tender for the area, said one person close to the Guinean government.

In the letter, addressed to Guinea's mining minister, Kerfalla Yansane, the Anglo-Australian giant sets out its rationale for suing Vale and Israeli billionaire Beny Steinmetz. Rio Tinto alleges the two colluded to rob it of the rights to one of the mining world's most coveted prizes, a roughly 600 square-kilometer iron-ore concession in the West African nation's Simandou Mountains.

"Rio Tinto wishes to clarify that this is a damages claim, and not a claim for reinstatement of the mining titles in question, nor in any way an action against the government of Guinea," wrote
Alan Davies,
the Rio Tinto executive in charge of the company's own Simandou project.

A Rio Tinto spokesman declined to comment. The mining arm of Mr. Steinmetz's family conglomerate, BSG Resources, has described the lawsuit as "baseless and bizarre," while Vale's chief executive,
Murilo Ferreira,
told The Wall Street Journal last month that it was "up to the accuser to show proof, under penalty of acting in bad faith."

Rio Tinto used to hold the rights to all four of Simandou's license blocks, a deposit that in its entirety is worth around $20 billion at current iron ore prices, according to an estimate by Sanford C. Bernstein analyst Paul Gait.

In 2008, former Guinean President Lansana Conté stripped the company of half its title—Simandou blocks 1 and 2—and later awarded them to BSG Resources. Vale then agreed to a separate deal with BSG Resources, worth $2.5 billion, to join its project.

But earlier this year Guinea's current government, headed by longtime Conté opponent
Alpha Condé
—accused BSG Resources of paying bribes to win the licenses, an allegation that the company has denied. Vale, meanwhile, has said any alleged wrongdoing in the contract award occurred before their involvement.

Last month, it stripped BSG Resources and Vale of its share of Simandou, putting blocks 1 and 2 back into play.

That decision reignited interest in Simandou from some of the world's biggest mining companies, who notified authorities they would be prepared to bid in a new tender for the blocks, according to people close to the Guinean government.

Against this backdrop, Rio Tinto's decision to sue Vale, Beny Steinmetz and BSG Resources set off alarm bells in the capital of Conakry, according to one person. Of particular concern was the fear that Rio Tinto's suit would discourage rivals from taking part in any future tender, said the person.

However, Mr. Davies, in his letter to Mr. Kerfalla, expressly says that the company would refrain from such a move.

"Rio Tinto will not challenge any decision by the Government of Guinea in this regard, nor seek to prevent or interfere with any legal process of reallocation of Simandou blocks 1 and 2 the government of Guinea may undertake, nor to challenge the title of any party who may come to acquire an interest in those blocks as a result of any such process," writes Mr. Davies.